1. Field of the Invention
The invention is based on an actuating drive for a control valve having a main piston which can be operated by oil pressure and having a pilot control system in effective connection with the main piston by means of a linkage, which pilot control system has at least one control orifice controlling the oil pressure at the main piston and at least one pilot piston arrangement which can be hydraulically operated.
2. Discussion of Background
An actuating drive for operating a control valve by which, for example, the steam supplied to a turbine is controlled, has a main piston which is subjected at one end to spring force and at the other end to oil pressure. When the oil pressure drops, the spring force reliably closes the control valve, which therefore cuts off the steam supply in every case. This ensures that the turbine cannot reach any uncontrollable operating condition if the oil pressure for the control valve should fail. In this design, an actuating signal is generated as an oil pressure by a central electrohydraulic converter which simultaneously operates a plurality of actuating drives. This involves comparatively long signal pipelines for the oil transmitting the actuating signal so that the necessary pipework volume has a negative effect on the dynamic behavior of the actuating drive.
The main piston of the actuating drive is connected via a linkage to a control orifice which interacts with a pilot piston arrangement and controls the flow of oil to the main piston. If a particular stroke characteristic of the actuating drive is desired, this can only be achieved by means of a complicated device--such as slotted links or cam discs with corresponding mechanisms--integrated in the linkage. The stroke of the actuating drive is generally monitored in operation and in this case, the monitoring is only possible by means of a relatively complicated measurement device. An additional disadvantageous effect is that the actuating drive function cannot, in this case, be made safe directly by means of a hydraulic oil safety circuit.